1. Field of Invention
The present invention relates to the field of control system implementation. In particular, the present invention describes a method and apparatus for resetting a process in a control system.
2. Description of Related Art
Semiconductor manufacturers use semiconductor wafers as the base for manufacturing integrated circuits. In one step of the manufacturing process, the wafers are put through a chemical-mechanical polishing step that leaves the wafers' surfaces covered in particles. Semiconductor manufacturers use double-sided wafer scrubbers (scrubbers) to clean the semiconductor wafers after being polished. Double-sided wafer scrubbers clean both sides of each wafer to remove these particles.
To reduce the risk of damaging the wafers, automated scrubbers have been developed. Scrubbers typically include a number of automated stations that operate together to clean wafers. Typically, to clean a wafer, the wafer must first be loaded from the cassette (a device for holding wafers), washed, brushed, dried, and then placed into another cassette. Typically, a scrubber cleans multiple wafers at the same time; one wafer being in each station. On occasion, one of the stations in the cleaning process will fail. Typically, a failure can occur because of a human operator (operator) error, a power surge, or a sensor failure. Although these types of failures occur rarely, the loss of one wafer can be very expensive. Therefore, some scrubbers have attempted to provide mechanisms for recovering from a failure during the cleaning process.
One system, the DDS200.TM. scrubber, available from OnTrak Systems, Inc., of Milpitas, Calif., allows the operator to shutdown the scrubber and then restart the cleaning process. However, this means that the entire machine is powered off and power is lost at each station. This may cause, for example, water, chemicals, and air to stop flowing. While the machine is shutdown, the possibility exists that a wafer will dry before being completely cleaned. Once a wafer dries, it cannot be cleaned, and the wafer will be lost.